U.S. Pat. No. 3,395,801 depicts a basic rotary filtration structure. U.S. Pat. Nos. 3,997,447 (to Breton et al) and 4,698,156 (to Bumpers), in particular, are illustrative of rotary filtration structures which Applicant's invention improves, involving high speed rotating micropore disc filteration devices. In U.S. Pat. Nos. 3,395,801, 3,997,447 and 4,698,156, a plurality of filter discs are mounted on a rotatable hollow shaft having a plurality of radial openings. The shaft is mounted for high speed rotation in a vessel or chamber which, unlike in a bowl centrifuge, remains stationary. The fluid suspension to be filtered is admitted directly into the vessel The filtration surfaces rotate through the fluid suspension, and filtrate flows through the surfaces into the core of the filter. The filtrate flows from the filter core through the radial openings in the shaft into the hollow of the rotating shaft, and then is conducted out the hollow for collection outside the vessel.
More particularly, as disclosed in U.S. Pat. Nos. 3,997,447 and 4,698,156 it is known to rotate through a fluid suspension a porous filtration element composed of a laminated structure comprised of combinations of porous layers in which the pores of each layer are in open communication. The filtration elements are usually constructed from sinterable metal or ceramic powders which have been sized and formed into laminated plates of interconnecting and communicating pores by powder metallurgy techniques or with fine screens having micron range pore sizes made using expanded metal technology or by selective dissolution techniques. Several preparation methods are disclosed in U.S. Pat. No. 3,997,447, which is incorporated herein by reference for all purposes. In the filtration elements, an interior layer of large pore sizes acts as a support and collecting layer for an outer confining layer of smaller pore sizes. The pore sizes may be submicron and larger. Thus, for example, an application of such filtration devices may be to remove minute particles from lake or other intake water used in steam injection plants or in nuclear power plants to reduce corrosion buildup in steam lines and reduce problems of disposal of particles in primary cooling loop steam which would be made radioactive. As disclosed in U.S. Pat. No. 3,997,447, the sintered metal powder filtration element conventionally is fabricated into structure such as annular plates, attached to a hollow shaft. Fluid separated from suspended matter in a fluid suspension through which the element is rotated flows from the outer to the inner layer of the element and courses through admitting slots provided in the hollow shaft to a conduit leading to the exterior of the shaft for collection at a remote point.
A difficulty with the rotating filter disc technology disclosed by U.S. Pat. Nos. 3,997,447 and 4,698,156, is that to be processed by the spinning filter discs, the fluid suspension must be in a processing zone which is between the shaft and the tips of the spinning microdiscs, and to gain access to the processing zone, the fluid suspension must flow radially inwardly past the tips against the flow direction of solids and fluids centrifugally cast radially outwardly from the shaft.
This circulation problem in high speed rotating disc filters is not present in such rotational filtration apparatus as represented by U.S. Pat. No. 2,038,921. The apparatus in U.S. Pat. No. 2,038,921 removes filtrate through a hollow rotary shaft like U.S. Pat. Nos. 3,997,447 and 4,698,156, but, unlike the discs of U.S. Pat. Nos. 3,997,447 and 4,698,156 which cut through fluid suspension in the vessel at high speeds, U.S. Pat. No. 2,038,921 rotates the filter units only exceedingly slowly; revolutions are measured in terms of hours (col. 4, line 75 to col. 5, line 2). The filter units are rotated slowly specifically to prevent agitation within the vessel when the filter is in operation (col. 2 line 59 to col. 3 line 1) while shaving off impurity laden surface layers of a filter cake builtup in situ for the filtration medium. Like U.S. Pat. Nos. 3,395,801 3,997,447 and 4,698,156, in U.S. Pat No. 2,038,921 the fluid suspension to be filtered is introduced directly into the vessel, but unlike them, the fluid suspension is introduced through the same perforated piping employed for the in situ laydown of the filter cake. The perforations are spread along the pipe aimed at the plate on which the filter cake is deposited, but the perforations do not reach near the shaft, avoiding filter cake buildup proximate the shaft.
Nor is the impedance in fluid suspension circulation that is the problem of high speed rotating discs encountered in bowl centrifuge filtration apparatus. In bowl centrifuges such as those of the type disclosed in U.S. Pat. Nos. 1,269,097 and 1,038,607, the filtration members are not spun through the fluid suspension; instead the fluid suspension and the filtration membranes both spin in the same direction inside the spun bowl. In U.S. Pat. Nos. 1,269,097 and 1,038,607, the suspension to be filtered is introduced through a radially widening axial conduit at the center of the bowl for discharge into at the base of the bowl into an open chamber to separate the heavier matter. Pathways are provided up from the open base chamber for flow of the clarified fluids through filtration membranes to a collecting annulus.
Most remote to the problem is filtration apparatus such as U.S. Pat. No. 4,243,536, in which no part of the filtration apparatus is rotary. In U.S. Pat. No. 4,243,536, the whole apparatus is stationary, and operates based on a cycloidal flow of liquid emitted tangentially from an inlet pipe near the center of the cylinder.